Copper etchant



Jan. 2, 1968 B. MILLER ETAL 3,361,674

COPPER ETGHANT Filed June 5, 1964 3 Sheets-Sheet 1 FIG. 3

0.4 3 i g 0.3 Q

0. E Q: 5 k 0.\ u

o 30 6O 90 I20 150 I80 COPPER ADDED GRAMS/L/TER INVENTORS B MILLER r. 0. SCHLABACH, JR.

ATTORNEY Jan. 2, 1968 B. MILLER ETAL 3,361,674

COPPER ETCHANT Filed June 5, 1964 5 Sheets-Sheet 2 2 POTENT/OMETR/C RECORDER I MICROSW/TCH MOTOR OR SOLENOID HCL SUPPLY FIG. 4

TOTAL MLS HC! l I I l l O 30 6O 90 I20 I50 I80 COPPER ADDED, GRAMS/L/TER United States Patent Ollice 3,361,674 Fatented Jan. 2, 1968 This invention relates to solutions for chemically etching copper.

Etching copper is of considerable technological importance particularly in the fabrication of electrical devices and elements such as printed wiring boards. The photoengraving art also has a significant interest in etching copper.

One of the most successful of the existing chemical etch solutions for copper relies on cupric chloride in chloride solution via the reaction:

The equilibrium constant is very favorable in suflicient chloride but the rate of etching falls oil quite rapidly in this medium and imposes a practical limit, usually on the order of half of theory. Electrolytic regeneration via anodic reaction to yield Cu with plating of copper at a relatively small and polarized cathode has been suggested as a means of extending the effective life of the etching solution. However, this technique, on a large scale, requires a sizable initial investment in equipment and high power consumption.

In principle, the continuous electrolytic technique offers significant advantages over the chemical methods of regeneration which have been proposed. Agents such as oxygen and hydrogen peroxide have been used in conjunction with necessary additions of acid to reoxidize Cu+, but these reagents are relatively active and unstable and necessitate batch processing. Air oxidation has been suggested and tried; however, it is inherently slow and is limited to spray work.

These and other difiiculties are in whole or part over come by the use of a soluble chlorate C; to oxidize the cuprous ion formed as a result of reaction (1). This presents a continuous regeneration process for the de sired cupric ion and the effective life of the solution is considerably lengthened.

According to the invention the etching proceeds principally through the continuous sequence:

Reaction (2) is aided by the presence of acid as is evident from the equation prescribing the over-all reaction:

Since the chlorate provides six electrons per mole and has a very low equivalent weight it is practical to provide a considerable reservoir of chlorate in the initial solution. Theoretically, one mole of chlorate is sufiicient to dissolve three moles of copper metal before the etch rate falls oil and the solution becomes exhausted.

If the total stoichiometric amount of acid (H conveniently added as HCl, is added to the initial solution, C1 and/or C10 gases may be liberated by side reaction.

While these reactions tend to be slow the products are noxious and may detrimentally attack parts of the Workpiece such as the organic resins of wiring boards. Accordingly, it is expedient to add the acid approximately as it is consumed. This can be done conveniently by monitoring the electrode potential of the solution and adding acid to maintain the value within an indicated range.

Where it is found to be inconvenient to make periodic acid additions the etchant solution of this invention will provide an unexpectedly long effective life without acid additions. The novel mechanism responsible for this alternative etching mode is given by the equation:

The etching capacity of this solution is again limited by the equivalence of 3 mols Cu/mol 010-; and also by the cupric ion: 3 mol Cu/mol Cu++ in the original solution. This is distinguished from the conventional use of cupric ion etches which have one-third of this capacity since there is, in that case, a one-to-one stoichiometry Cu/Cu++. Furthermore, the equilibrium of reaction (4) is especially advantageous due to the formation of insoluble Cu (OH) Cl.

Reaction (4) also requires the initial presence of excess chloride which is conventional and typically added as NaCl. The addition of NaCl as a complexing agent is also helpful in the previous case even though chloride is obtained from the HCl additions.

The understanding of these and other aspects of the invention will perhaps be aided by the following detailed description. In the drawing:

FIG. 1 is a schematic representation of an etching apparatus used to demonstrate the principles of this invention;

FIG. 2 is a schematic representation of an etching cell with an associated servo mechanism for automatically controlling the acid additions to the cell in accordance with the novel regenerative mechanism represented by reaction (3) of this invention;

FIG. 3 is a plot of the etch rate vs. time (expressed as copper etched, gms./liter) for three characteristic solutions formulated according to this invention;

FIG. 4 is a plot of HCl additions vs. time (copper etched) for the three etch solutions of FIG. 3 illustrating the stoichiometric behavior of the reaction expressed by equation (3);

FIG. 5 is a plot of etch rate vs. time (copper etched) for a solution operated according to reaction (3), a solution operating according to reaction (4), and a typical prior art etchant solution; and

FIG. 6 is a plot of etch rate vs. time (copper etched) for three solutions of this invention having dilferent ini tial ClO content showing the direct dependence of effective life on the concentration of this constituent.

Etch solutions were tested using the apparatus of FIG. 1. The solution being tested was placed in a trimmed 400- ml. beaker 1% provided with a 4-blade paddle stirrer ll driven by motor 12. Etch time measurements were made on a copper plated wire 13. The wire was a 1 cm. section of 64-mil platinum wire having a copper plate 0.96 mil in thickness. The terminals of the wire section were provided with polyethylene seals 14 to define a constant exposed area. The plated wire 13 was suspended in a glass rod 15 which is supported in the solution by a collar 35 and bearing 17. The support rod 15 is rotated with a belt 18 attached around the collar 16 and driven by the stirrer 3 shaft so that the etching action on the plated section is uniform.

The acidity of the solution was continuously monitored .by a pair of wire electrodes 19 and 20 supported in glass sleeves 21 and 22. Electrode 19 is a silver wire and monitors a silver-silver chloride couple. This effectively serves as a reference electrode since the total shift due to molality and activity coefficient from fresh to.exhausted etchant states is less than 75 millivolts for the silver potential in the solutions considered. Acid additions are automatically controlled by a solenoid controlled stopcock 23 on burette 24.

The details of the monitor and servo assembly are shown in FIG. 2. Voltages are measured directly on a potentiometric recorder (Varian G or Mosely 3S) equipped with a :microswitch activated by the recorder pen assembly when the potential exceeded a preselected voltage value. The switch in turn controls the motoror solenoidoperated burette stopcock 23 to add concentrated ('12 M) hydrochloric acid in' an amount necessary to bring the platinum electrode potential up to the value where the switch would be reversed and the stopcock closed.

As is evident from the following data the solutions forming the basis for this invention were used successfully both with and without acid additions.

In FIG. 3 the etch rate in mils/minute is plotted vs. the amount of copper dissolved in grams/liter. The abscissa reflects the effective life of the solution. Data is given for three different etch solutions with compositions These data suggest the dependence of the capacity or effective life of the etching solution on the original concentration of NaClO since each solution exhibits approximately the same copper capacity.

FIG. 4 plots the optimum schedule of acid additions for the same three solutions used to obtain the data of FIG. 3.

The addition of acid as required by the schedule up to the stoichiometric maximum (indicated by the line) provides for the maximum life of the solution as measured by the NaClO content.

FIG. 5 is a plot of etch rate over the eifective life of three etch solutions. Curve D is a solution formulated according to the prior art relying simply on the capacity of cupric chloride to oxidize the copper. The effective life is considerably limited. Curves E and F give results for an etch solution formulated according to this invention and based upon the capacity of NaClO as an oxidant to extend the effective operating life of the solution. The composition of the etchant used for the data of curves E and F corresponds to that of curve C in FIG. 3 (0.6 M CuCl 1.2 M NaCl, 1.5 M NaClO The etch mode of curve B did not include acid additions. The data for curve F was obtained using periodic additions up to the stoichiometric amount of HCl. The etch procedures used to obtain the data for both curves E and F are clearly more effective both in terms of the effective life of the solution and the over-all uniformity of the etch rate.

FIG. 6 is a plot showing data similar to that given in the preceding figures. This figure illustrates the effective life of three different etch solutions and the dependence of the capacity of the solution on the concentration of NaC1O Curves G, H and I were obtained using differing concentrations of NaClO according to the following schedule:

Curve G CUIVB H Curve .1

M 011012 2.0 2.0 2.0 M NaCO; 0.5 1.0 2.0 M NaCl 2. 0 2.0 2.0

any soluble chlorate is effective within the concentration limits prescribed. All the solutions mentioned herein are understood to be aqueous solutions.

The CuCl may likewise be included in widely varying concentrations from 0.1 M to the solubility limit which is approximately 4 M. The concentration of CuCl determines the maximum etch rate assuming a favorable equilibrium and the etch rate may vary significantly depending upon the application for which the etch process is intended.

The foregoing constituents, CuCl and NaClO are the essential ingredients according to this invention. However it is often found desirable initially to include an excess chloride concentration to provide a more favorable equilibrium by complexing the cuprous product intermittently formed from dissolved copper. Again, the amount of chloride included for this purpose is largely a matter of choice in the specific behaviour of the solution. Consequently this constituent is generally prescribed .in an amount of zero to saturation. The chloride is conveniently added as NaCl, however, other soluble chlorides may be used as well. A useful concentration range is 0.1 M. to 5 M.

Various other modifications and extensions of this invention will become apparent to those skilled in the art. All such variations and deviations which basically rely on the teachings through which this invention has advanced the art are properly considered within the spirit and scope of this invention.

What is claimed is:

1. An etch solution consisting essentially of water having the following essential ingredients in the approximate quantities indicated:

sodium chlorate0.1 molar to 3 molar cupric chloride0.1 molar to 4 molar sodium chloride-0.1 molar to 5 molar.

References Cited UNITED STATES PATENTS 3,231,503 1/1966 Laue 252-79.1 2,291,202 7/1942 Bassett et al 252-142 X 2,796,334 6/1957 Robinson 252792 X 2,856,275 10/1958 Otto- 252792 X 2,908,557 10/1959 Black et a1. 'l343 X OTHER REFERENCES Marconi Review: The Etching of Printed Circuit Boards, Clark, vol. 24-25, l9611962, pp. 134-452.

LEON D. ROSDOL, Primary Examiner.

JULIUS GREENWALD, SAMUEL H. BLECH,

Examiners.

S. D. SCHNEIDER, Assistant Examiner. 

1. AN ETCH SOLUTION CONSISTING ESSENTIALLY OF WATER HAVING THE FOLLOWING ESSENTIAL INGREDIENTS IN THE APPROXIMATE QUANTITIES INDICATED: SODIUM CHLORATE-0.1 MOLAR TO 3 MOLAR CUPRIC CHLORIDE-0.1 MOLAR TO 4 MOLAR SODIUM CHLORIDE-0.1 MOLAR TO 5 MOLAR. 